Thyroid

Ultrasound of thyroid gland: normal anatomy and sonographic features

Embryologically, the primordium of thyroid gland appears as a diverticulum arising from the endoderm at dorsum of the tongue (foramen cecum), between the first and second pharyngeal pouches.

The downward migration of the thyroid diverticulum forms the thyroglossal duct along the midline anteriorly to the hyoid bone and thyroid cartilage.

Thyroglossal duct cysts appear due to failure of involution of a portion of the thyroglossal duct.

Remnant of thyroid tissue (due to sequestration of thyroid tissue along the thyroglossal duct) and ectopic thyroid gland (due to incomplete descent of the thyroid into the lower neck) may be found anywhere this path of descent.

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Fig. 1 - Illustration depicting the path of the thyroid descent. Thyroglossal cysts occurs along the migratory pathway of the thyroid gland and are always close to midline.

SCANNING TECHNIQUE

Before we talk about anatomy, to properly identify and scan the thyroid gland, some technical conditions must be fulfilled (Table 1).

PREPARATION: no specific preparation is required

POSITION: patient should be in the supine position with the neck slightly hyperextended by placing a pillow under the shoulders. Patient should avoid swallowing or talking during exam.

PROBE: High-frequency transducers (7-12Mhz), which has the best spatial resolution, should be used. In cases of large thyroid glands ( goiter), short necks or obese patients a lower frequency probe may be necessary.

STANDARD SECTIONS: Both lobes must be scanned individually in the transverse and longitudinal plane. Any finding should be evaluated in both planes by rotating the transducer 90 degrees.

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Table 1. Thyroid ultrasound technique

NORMAL THYROID: ANATOMY AND SONOGRAPHY FEATURES

The thyroid is an “H” shaped gland located in the visceral space of the infrahyoid neck, about 1-3 cm above the breastbone. It is divided into two lobes connected by a narrow isthmus that crosses anteriorly the trachea. The anatomical relation between the thyroid and the neighborhood are the following:

- Anteriorly, the thyroid is covered by the thin strap muscles (sternohyoid, sternothyroid and omohyoid).

- Anterolaterally, we have the more bulky sternocleidomastoid muscles.

- Posteriorly to each thyroid lobe we have the longus colli muscles.

- Laterally to each lobe we have the common carotid arteries and jugular veins.

Fig. 4 - The esophagus is usually seen neighbooring the posterior and medial margin o left thyroid lobe. Note the "gut signature" also observed in the esophagus as different wall layers with alternating hypoechoic and hyperechoic appearance (arrows on zoomed upper right image). Note the echogenic foci (*) within the esophagus lumen representing air. Swallowing can help differentiating it from neck lesions.

THYROID ECHOGENICITY AND ECHOTEXTURE

The normal thyroid parenchyma is hyperechoic (bright) compared with the prethyroid muscles (strap muscles), with fine and homogeneous echoetexture and also a smooth echogenic capsule.

Fig. 6 - In additionto comparing the thyroid echogenicity with the strap muscles, we should compare its echogenicity with the submandibular gland. Both glands have similar echogenicity. Note the capsule depicted as a thin echogenic structure anteriorly lining the thyroid (arrowhead).

THYROID MEASUREMENTS

Measurement of thyroid lobes involves three measurements: length(L), depth(D) and width(W). The thyroid volume is calculated using the formula for a prolatte elipse: p/6(LxDxW). Normal ultrasound values for thyroid dimensios are 13-18 mm for width, 16-18 mm for depth and 45-60 mm for length with normal isthmus about 2-6 mm deep. The volume of a normal thyroid in adults is still source of debate, however, we can consider a normal volume of 7.7-19.1 cm3 in men and 4.8-15.1 cm3in women.

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Fig. 7 - Both the width and depth are measured in a transverse view of thyroid lobe (left image).The width (W) is measured from a horizontal imaginary line drawn along the lateral edge of the trachea to the lateral thyroid border. Depth (D) is measured as the maximum antero-posterior diameter at the middle third of thyroid lobe. Length (L) is measured in a sagittal view of the lobe (right image) as the maximum longitudinal distance from upper to lower thyroid poles. Bottom image shows the measurement of isthmus (i) anteroposterior diameter.

THYROID ARTERIAL SUPPLY

The blood supply of the thyroid gland is from the paired superior and inferior thyroid arteries (Fig. 8). The superior thyroid arteries usually are the first branches of external carotid artery. It is divided into an anterior branch, which runs toward the isthmus to anastomose with the contralateral artery and a posterior branch, descending through the back of the lobe to anastomose with an ascending branch of the inferior thyroid artery.The inferior thyroid arteries are the largest branches of the thyrocervical trunk, which arises from the subclavian artery. In about 10%, a thyroid ima artery arises from the brachiocephalic trunk, the arch of aorta or from the right common carotid. This small artery ascends anteriorly to the trachea and enters tthe inferior border of the isthmus (Fig. 8).

The reference values for doppler ultrasound parameters of thyroid arteries in healthy individuals have been described (Table 3). Usually the average systolic peak velocity (SPV) in STA is greater than in ITA, probably because of their afferent vessels (external carotid artery supplying the STA and thyrocervical trunk supplying the ITA). Another important observation is about the impedance indices, which are higher in STA. Perhaps these may be explained by the fact that the efferents vessels from STA supply not only thyroid parenchyma, but also cervical structures with high impedance indices, such as muscles and the larynx.

Board certified Brazilian radiologist, co-founder of the Salus Sonography project and Medultra smartphone app. Currently works as an interventional and general radiologist in private practice, São Paulo, Brazil.